Universität zu Köln

Seifried, D., Beuther, H., Walch, S., Syed, J., Soler, J. D., Girichidis, P. and Wuensch, R. (2022). On the accuracy of H I observations in molecular clouds - More cold H I than thought? Mon. Not. Roy. Astron. Soc., 512 (4). S. 4765 - 4785. OXFORD: OXFORD UNIV PRESS. ISSN 1365-2966

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Abstract

We present a study of the cold atomic hydrogen (H I) content of molecular clouds simulated within the SILCC-Zoom project for solar neighbourhood conditions. We produce synthetic observations of H I at 21 cm, including H I self-absorption (HISA) and observational effects. We find that H I column densities, N-H I, of greater than or similar to 10(22) cm(-2) are frequently reached in molecular clouds with H I temperatures as low as similar to 10 K. Hence, HISA observations assuming a fixed H I temperature tend to underestimate the amount of cold H I in molecular clouds by a factor of 3-10 and produce an artificial upper limit of N-H I around 10(21) cm(-2). We thus argue that the cold H I mass in molecular clouds could be a factor of a few higher than previously estimated. Also, N-H I PDFs obtained from HISA observations might be subject to observational biases and should be considered with caution. The underestimation of cold H I in HISA observations is due to both the large H I temperature variations and the effect of noise in regions of high optical depth. We find optical depths of cold H I around 1-10, making optical depth corrections essential. We show that the high H I column densities (greater than or similar to 10(22) cm(-2)) can in parts be attributed to the occurrence of up to 10 individual HI-H-2 transitions along the line of sight. This is also reflected in the spectra, necessitating Gaussian decomposition algorithms for their in-depth analysis. However, also for a single HI-H-2 transition, N-H I frequently exceeds 10(21) cm(-2), challenging one-dimensional, semi-analytical models. This is due to non-equilibrium chemistry effects and the fact that HI-H-2 transition regions usually do not possess a one-dimensional geometry. Finally, we show that the H I gas is moderately supersonic with Mach numbers of a few. The corresponding non-thermal velocity dispersion can be determined via HISA observations within a factor of similar to 2.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Seifried, D. UNSPECIFIED UNSPECIFIED UNSPECIFIED Beuther, H. UNSPECIFIED UNSPECIFIED UNSPECIFIED Walch, S. UNSPECIFIED UNSPECIFIED UNSPECIFIED Syed, J. UNSPECIFIED UNSPECIFIED UNSPECIFIED Soler, J. D. UNSPECIFIED UNSPECIFIED UNSPECIFIED Girichidis, P. UNSPECIFIED UNSPECIFIED UNSPECIFIED Wuensch, R. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-680599
DOI:	10.1093/mnras/stac607
Journal or Publication Title:	Mon. Not. Roy. Astron. Soc.
Volume:	512
Number:	4
Page Range:	S. 4765 - 4785
Date:	2022
Publisher:	OXFORD UNIV PRESS
Place of Publication:	OXFORD
ISSN:	1365-2966
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language NARROW SELF-ABSORPTION; STAR-FORMATION; SILCC-ZOOM; INTERSTELLAR CLOUDS; PHYSICAL CONDITIONS; MAGNETIC-FIELDS; HIGH-RESOLUTION; DATA RELEASE; DARK CLOUDS; LINE SURVEY Multiple languages Astronomy & Astrophysics Multiple languages
URI:	http://kups.ub.uni-koeln.de/id/eprint/68059